(1). Field of the Invention
The invention relates to the general field of semiconductor integrated circuits with particular reference to planarization techniques.
(2). Description of the Prior Art
FIG. 1 shows a schematic view of a portion of a partially completed integrated circuit. Lying on substrate 10, which could be a silicon substrate or a dielectric layer such as field oxide or BPSG (boro-phospho-silicate glass), are two metal lines 12 which run in a direction normal to the plane of the figure. An IMD (inter-metal dielectric) layer 13 has been deposited over lines 12 in preparation for forming the next level of wiring. Because lines 12 are relatively close together, narrow gap 14 has been left in the top surface of 13. On average, the width of such a gap would be between about 0.03 and 0.5 microns.
Referring now to FIG. 2, planarizing layer 15, typically SOG (spin-on glass) has been deposited over layer 13. As shown, gap 14 (of FIG. 1) has been partially filled in but a large pocket of gas 16 has become trapped between layers 13 and 15. This is a situation to be avoided because a subsequent etchback process could remove the top SOG layer to open up a new gap which, in later oxide deposition steps, results in a rough surface topology. This, in turn, can give rise to metal filaments. It is therefore important to be able to fully fill gap 14 and not leave any trapped gas behind.
A number of ways of improving the filling of narrow gaps have been proposed in the prior art. For example, Kalnitsky et al. (U.S. Pat. No. 5,435,888 July 1995) apply a very thin layer of SOG that is able to flow into (and presumably fill) narrow gaps. This is followed by a layer of CVD deposited oxide (such as TEOS) and, finally, by a second layer of SOG which is also thin.
Chen et al. (U.S. Pat. No. 5,567,660 October 1996) apply SOG to a stationary wafer. The wafer is then caused to spin relatively slowly, just enough to make it begin to spread out over the surface. Once the surface is evenly covered, the spinning speed is increased. By this time the SOG viscosity has increased somewhat (due to drying) so the overall thickness of the SOG coating is not affected by the increased spinning rate. However, the centrifugal force associated with the higher speed spin causes the SOG to better fill in the gaps.
Wang (U.S. Pat. No. 5,554,567 September 1996) improves the adhesion of a SOG layer by providing a short heat treatment, in vacuo, after the SOG has been cured. A second layer of SOG is then deposited without breaking vacuum.
The present invention teaches a method (for fully filling gaps such as 14 in FIG. 1) which is quite different from those described above but which is highly effective and easy to implement.